Christian Solidarity Worldwide (CSW) wants a judicial review of the case of Alimujan Yimit.
He was convicted in 2009 and is serving a prison sentence for "illegally providing state secrets to foreign nationals".
He is a Uyghur Christian, part of a small community from Xinjiang in north-western China.
CSW believes that his detention is related to earlier charges he faced relating to 'illegal' religious activities as the head of an unregistered church.
The United Nations Working Group on Arbitrary Detention has agreed that Alimujan Yimit been arbitrarily detained.
Xinjiang is an autonomous region of China that has been affected by China's crackdown on religious activities, including cross removals from churches and banning Muslim students from fasting during Ramadan.
CSW also received reports that last month authorities had been asking school pupils in the region about their families' religious practices.
It's Chief Executive Mervyn Thomas said: "CSW has campaigned on Alimujian Yimit's case for several years and continues to be concerned about his conviction and detention, which the UN has declared to be arbitrary.
"We believe the charges against him are groundless and that he is being penalised for his peaceful religious activities as an unregistered church leader."
"We urge the authorities to re-assess his case thoroughly, impartially and without delay, with a view to securing his unconditional release," he said.
"We further call on the government to ensure that any persons found responsible for Yimit's wrongful imprisonment and/or ill-treatment in detention are held accountable and to ensure that Yimit's current conditions in detention comply with international standards."